Known person-identifying methods which are e.g. based on face recognition as described in German patent 44 06 020 only compare single photographs of the person to be identified with reference pictures. Thus only static information is processed in said methods. Such methods have the drawback that an access control operating with such a method can easily be deceived by showing a photo or a mask-like picture of the person.
Therefore, various methods have been developed for reducing the risk of such deceptions.
For instance, it is known from German Utility Model 2 950 660 that the spectral range of the light used for identifying a person is limited by an IR filter disc to a range in which a photo or a mask-like picture has reflectance characteristics which differ from those of an actually recorded face. Such an IR filter disc has the drawback that the image quality is reduced thereby and that the demands made on the quality of the identifying process increase. On the other hand, there is the risk that an image medium is found with reflectance characteristics in the IR range that are similar to the reflectance characteristics of a person's face in the IR range, whereby a deception is again possible with such an image medium.
Furthermore, the prior art discloses methods in which the picture of the person to be identified is taken with a stereo imaging system. For instance, such a method is used in the face recognizing system described in the article “In-Your-Face Security” in PCWeek of Mar. 26, 1997. Other methods for detecting the three-dimensional structure of a face either fully or in part are laser triangulation or the so-called shape-from-shading methods disclosed by Attick, Griffin and Redlich in Neural Computation 8, 1321-1340, 1996; these make it possible to draw conclusions from the gray-level curve at a given direction of illumination with respect to the depth information of the object viewed. Although deceptions in the case of which an image is held in planar fashion in front of the recording system can be detected with such methods, deceptions with a cylindrically bent image or a three-dimensional mask cannot be recognized as such.
Moreover, the prior art discloses methods in which the pictures of the person to be identified are taken by means of thermographic methods; see e.g. German patent 4 009 051. In this instance, the radiant heat emitted by the person to be identified is detected and compared with a given reference value for identifying the person. This method, however, has the drawback that the devices for carrying out the methods are very expensive, in particular the thermographic camera used for image-recording the person to be identified. Thus thermographic methods cannot be used in access controls which are to be used in many places, e.g. in cash dispensers for withdrawing money and/or in banking transactions.